Figure 3.

Analysis of the sensitivity of the measured HP ¹³C Lac signal to specific metabolic parameters. Simulations were performed using a three-pool model representing the ¹³C-labeling kinetics of Glc, Pyr, and Lac in the brain (a). The model is characterized by three metabolic fluxes: the cerebral metabolic rate of glucose (CMR_Glc), the conversion between pyruvate and lactate through lactate dehydrogenase (V_LDH) and the exchange between blood lactate and brain tissue lactate (V_out). (b) A typical step input function for glucose fractional enrichment reaching 70% after 14 s was used. (c) The sensitivity of the Lac ¹³C concentration relative to the CMR_Glc was assessed by varying the CMR_Glc from 0.25 micromole/g/min to 1 µmol/g/min, with a nominal Lac pool size of 4 µmol/g. (d) The sensitivity of the Lac ¹³C concentration relative to the Lac pool size was assessed by varying the Lac pool size from 2 µmol/g to 8 µmol/g, with a nominal CMR_Glc of 4 µmol/g. In (e–g), the physical parameters influencing the measured ¹³C HP signal were included in the differential equations of the model (T₁ decay and repetitive RF pulsing). The ¹³C Lac signal is shown for a varying CMR_Glc with a fixed endogenous Lac concentration (f) or for varying endogenous Lac concentration with a fixed CMR_Glc (g).